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OpenMail automatically groups related emails into threads using standard RFC 2822 email headers.

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How it works

When an email arrives or is sent, we resolve which thread it belongs to:
  1. Check In-Reply-To header - If it matches an existing message’s Message-ID, the email joins that thread.
  2. Check References header - If any value matches an existing message’s Message-ID, the email joins that thread.
  3. No match - A new thread is created.

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Outbound replies

When you send with a threadId, we automatically set:
  • In-Reply-To → the Message-ID of the last message in the thread
  • References → all Message-ID values from the thread
This ensures your reply threads correctly in the recipient’s email client (Gmail, Outlook, etc.). We also append a quoted copy of the previous message to the body by default (the familiar On … wrote: block with >-prefixed lines). Pass includeQuote: false to send only your reply text.

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Thread context

Fetch all messages in a thread via GET /v1/threads/:id/messages. Messages are returned in chronological order, giving your agent the full conversation history to inform its next action.

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Read/unread tracking

Every thread has an is_read flag that tracks whether your agent has processed it. This prevents agents from reprocessing the same emails.

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How it works

  • New inbound threads start as unread (is_read: false)
  • Sending a reply automatically marks the thread as read
  • Your agent explicitly marks threads as read via PATCH /v1/threads/:id
1

Poll for unread threads

Fetch only threads your agent hasn’t processed yet.
2

Process the thread

Fetch messages, run your agent logic, send a reply if needed.
3

Mark as read

After successful processing, mark the thread as read so it won’t appear on the next poll.
If your agent sends a reply, this happens automatically.
If your agent uses webhooks or WebSockets, the same pattern applies — mark the thread as read after handling the event. Then use ?is_read=false as a safety net to catch any events missed during downtime.